Allow assessments to be edited by lecturer after submissions have started

When an assignment in Ultra has been viewed by any student, it is no longer possible for a lecturer to delete or re-upload an attached file that is in a separate block. The lecturer also cannot create new blocks in the assessment (the sign doesn't appear).

This is a problem for the following reasons:

it's not uncommon to want to update a file attached to an assessment piece, even (and perhaps especially) after students have had a chance to look at it.

it's not made clear to the lecturer why they are unable to delete the file.

it's still possible to edit the text block, which can contain uploaded files, so the academic integrity of "not changing an assignment after students have seen it" is not satisfied anyway.

it's not possible for the lecturer to see which students have viewed the assessment, so they can't know who is stopping them from editing the assessment.

it's not possible for the lecturer to 'reset' their viewing status (there's no submitted attempt to delete).

it's not possible for even an administrator to reset their viewing status except by unenrolling/reenrolling the student, which would delete any assessment attempts for all other assignments in the subject.

This is a significant issue that several of our lecturers have complained about, but I've been a little slow in investigating it thoroughly. It's quite a high priority, however, especially as we roll out significantly more Ultra courses in the coming year.